The present invention relates to silicon carbide single crystals, and in particular, relates to methods of forming high purity silicon carbide single crystal wafers that are at least 100 millimeters (mm) in diameter. Because of the relationship between English and Metric units (e.g., 25.4 mm=1 inch), such wafers are also referred to as “four inch” wafers.
The production of integrated circuits for many applications, such as RF devices, requires a substrate on which electronic devices can be built and connected to one another. Historically, sapphire was used as substrate material for microwave devices because of its high resistance to current flow. Sapphire has the disadvantage, however, of limiting the types of semiconductor layers that may be fabricated on the substrate with appropriate crystal lattice matching for proper device operation.
Silicon carbide (SiC) has a combination of electrical and physical properties that make it an attractive semiconductor material for high temperature, high voltage, high frequency and high power electronic devices. These properties include a 3.0 electron-volt (eV) bandgap (6H), a 4 Megavolt per centimeter (MV/cm) electric field breakdown, a 4.9 W/cmK thermal conductivity, and a 2×107 centimeter per second (cm/s) electron drift velocity. Silicon carbide is also particularly useful in its ability to be made conductive by doping or semi-insulating by various processing techniques. These qualities make silicon carbide a material of choice for a vast array of electronic applications.
Silicon carbide is, however, a difficult material to work with because it can crystallize in over 150 polytypes, some of which are separated from one another by very small thermodynamic differences. Furthermore, because of silicon carbide's high melting point (over 2700° C. under high pressure), many processes for working silicon carbide, including epitaxial film deposition, often need to be carried out at much higher temperature than analogous reactions in other semiconductor materials.
In one sense the commercial or manufactured synthesis of silicon carbide—typically for use as an abrasive—has been carried out for well over a century, with growth of polycrystalline silicon carbide being recognized by Acheson in 1885. The genesis of growth for electronic purposes, however, was initiated much later, being generally attributed to the development of the “Lely” method (named after its inventor) in 1955. In 1978, the first seeded sublimation techniques, which are also referred to as modified Lely techniques, were carried out, and by the late 1980s, silicon carbide technology was being rapidly commercialized, primarily by the assignee of the present invention.
In a seeded sublimation technique, a seed crystal and a source powder are both placed in a reaction crucible which is heated to the sublimation temperature of the source and in a manner that produces a thermal gradient between the source and the marginally cooler seed crystal. The thermal gradient encourages vapor phase movement of materials from the source to the seed followed by condensation upon the seed and the resulting bulk crystal growth. The method is also referred to physical vapor transport (PVT).
In a typical silicon carbide growth technique, the crucible is made of carbon and is heated by induction or resistance, with the relevant coils and insulation being placed to establish and control the desired thermal gradients. The source powder is silicon carbide, as is the seed. The crucible is oriented vertically, with the source powder in the lower portions and the seed positioned at the top, typically on a seed holder; see U.S. Pat. No. 4,866,005 (reissued as No. RE34,861). These sources are exemplary, rather than limiting, descriptions of modern seeded sublimation growth techniques.
Other techniques incorporate the silicon carbide seed, but use gases rather than SiC solids, as the source materials. Examples include commonly assigned U.S. Pat. No. 6,824,611 and U.S. Published Application No. 20050120943, the contents of each of which are incorporated entirely herein by reference.
From a practical standpoint, increasing the rate at which large single crystals of silicon carbide can be grown, increasing the diameter to which they can be grown, and reducing the defect density in the large crystals remain necessary and desired goals.
Although the density of structural defects in silicon carbide bulk crystals has been continually reduced in recent years, relatively high defect concentrations still appear and have been found to be difficult to eliminate. These can cause significant problems in limiting the performance characteristics of devices made on the substrates, or in some cases can preclude useful devices altogether. For example, a typical defect density in some commercially available silicon carbide wafers can be on the order of 100 per square centimeter (cm−2). A megawatt device formed in silicon carbide, however, will require a defect-free area on the order of 0.4 cm−2. Thus, obtaining large single crystals that can be used to fabricate large surface area devices for high-voltage, high current applications remains difficult.
Although occasionally named differently, the most common defects in silicon carbide bulk crystals are generally referred to as micropipes and hexagonal voids. A micropipe is a hollow core super-screw dislocation with its Burgers vector lying along the c-axis. A number of causes have been proposed or identified for the generation of micropipes. These include excess materials such as silicon or carbon inclusions, extrinsic impurities such as metal deposits, boundary defects, and the movement or slippage of partial dislocations. See e.g., Powell et al., Growth of Low Micropipe Density SiC Wafers, Materials Science Forum, Vols. 338-340, pp 437-440 (2000).
Hexagonal voids are flat, hexagonal platelet-shaped cavities in the crystal that often have hollow tubes trailing beneath them. Some evidence shows that micropipes are associated with hexagonal voids. A relatively recent discussion of such defects (exemplary and not limiting) is set forth in Kuhr et al., Hexagonal Voids and the Formation of Micropipes During SiC Sublimation Growth, Journal of Applied Physics, Volume 89, No. 8, page 4625 (April 2001).
Recent research indicates that problems in the bulk crystals produced in a seeded sublimation technique can originate with the seed itself and the manner in which it is physically handled; e.g., Sanchez et al., Formation of Thermal Decomposition Cavities in Physical Vapor Transport of Silicon Carbide, Journal of Electronic Materials, Volume 29, No. 3, page 347 (2000). Sanchez uses the term “micropipe” to describe, “approximately cylindrical voids with diameters in the range of 0.1 μm to 5 μm that form at the core of superscrew dislocations aligned parallel or nearly parallel to the [0001] axis” Id. at 347. Sanchez refers to larger voids (“diameters from 5 μm to 100 μm”) as, “thermal decomposition cavities,” and opines that micropipes and thermal decomposition cavities arise from different causes. Id.
Based on this hypothesis and his experimental work, Sanchez suggests that migration of “silicon rich vapor” from the back (opposite to growth) surface of a seed crystal into the growth system, causes thermal decomposition cavities that generate micropipes in the seed and then in the growing crystal. Sanchez suggests, “a continuous diffusion barrier for silicon bearing species,” will reduce or eliminate such cavities. Sanchez describes a carbonized sucrose barrier for this purpose, but admits that the technique is, “not entirely reproducible.” Id. at 352.
Although sublimation growth from silicon carbide powder has remained a technique of choice as the commercial demand for silicon carbide has expanded, similar techniques that incorporate seeds, but not necessarily sublimation, have also come into favor. As well-understood by those of ordinary skill in this art, seeded growth techniques can offer advantages in initiating and maintaining polytype control in silicon carbide growth.
Several reasons, some intrinsic, some theoretical, and some practical, have encouraged seeded growth techniques other than conventional sublimation.
As one reason, sublimation growth requires a silicon carbide source powder. As a consequence, growth of larger crystals can require either a large initial charge of the silicon carbide source powder, or a replenishment system for the silicon carbide powder.
As another factor, much commercially available silicon carbide powder feedstock, although generally high-quality, still may contain some contaminants that preclude it from satisfactory incorporation where ultra high purity growth is desired or necessary.
As another factor, as silicon carbide source powder sublimes, the composition (concentration) of the initial source powder will tend to change. For example, because the amount of silicon species in sublimation vapor is at least the same as or greater than the amount of carbon species, the composition of the source powder gradually changes, with the amount of carbon increasing. This tends to change the partial pressure of the species in the vapor over time. This in turn can make polytype control more difficult.
As another factor, if the powder source needs to be replenished, on-going sublimation growth must be stopped and the necessary apparatus disassembled and reassembled following addition of new source material. This tends to reduce the practical rate at which large crystals can be grown.
Accordingly, alternatives to sublimation growth that nevertheless incorporate the advantages of seed crystals continue to be developed.
Because growth on a seed from vapor phase source materials requires carbon-containing vapors and silicon-containing vapors, most of these non-sublimation techniques (i.e. those that do not initiate with silicon carbide as a source powder) either supply or generate the species from sources other than silicon carbide powder.
Nevertheless, when such techniques incorporate a seed, the relationships between and among the seed, the seed holder, and the remainder of the system continue to have all of the thermodynamic factors that can cause problems—or if addressed properly can solve problems—in seeded sublimation growth. Stated differently, the thermodynamic factors surrounding the seed and the seed holder are essentially the same regardless of the source (silicon carbide powder or otherwise) of the vaporized silicon-containing and carbon-containing species.
Therefore, improvements in the seed crystal, in the seed holder, and in the relationship between the seed and the seed holder offer advantages in substantially all types of seeded growth, and in order to continue to provide improvements in the quality of single crystal silicon carbide bulk crystals, and to reduce the defect density, the source of defects at the seed must be identified and successfully addressed.